1. Technical Field of the Invention
The present invention relates generally to electric rotating machines that include a plurality of multi-phase stator coils. More particularly, the invention relates to an electric rotating machine that has an improved arrangement of two three-phase stator coils for reducing magnetic noise and torque ripple.
2. Description of the Related Art
A conventional three-phase electric rotating machine includes a stator and a rotor. The stator includes a hollow cylindrical stator core and a three-phase stator coil wound around the stator core. The rotor has a plurality of magnetic poles that are formed with permanent magnets and are so arranged that the polarities of the magnetic poles alternate between north and south in the circumferential direction of the stator core. In such an electric rotating machine, magnetic noise and torque ripple are generally caused by the overlapping of harmonic components with sine-wave drive currents supplied to the phase windings of the three-phase stator coil. More specifically, the magnetic noise and torque ripple are mainly caused by the sixth-order harmonic components of the sine-wave drive currents. In addition, the wavelength of the sixth-order harmonic components is equal to π/3.
Japanese Unexamined Patent Application Publication No. H4-26345 discloses an automotive alternator that can reduce the magnetic noise and torque ripple caused by the sixth-order harmonic components. More specifically, the stator of the alternator includes a first three-phase stator coil and a second three-phase stator coil. Each of the first and second three-phase stator coils is comprised of three phase windings. Moreover, the first and second three-phase stator coils are wound on the stator core so that each of the phase windings of the first three-phase stator coil is offset by an electrical angle of π/6 from a corresponding one of the phase windings of the second three-phase stator coil. That is, a phase difference of π/6 is provided between the corresponding pairs of the phase windings of the first and second three-phase stator coils. Since the phase difference of π/6 is equal to half the wavelength (i.e., π/3) of the sixth-order harmonics components, the magnetic noise and torque ripple caused by the sixth-order harmonic components of sine-wave drive currents supplied to the phase windings of the first three-phase stator coil can be offset by those caused by the same of sine-wave drive currents supplied to the phase windings of the second three-phase stator coil. Consequently, the total magnetic noise and torque ripple generated in the automotive alternator can be reduced.
FIG. 12A illustrates the torque ripple generated in the conventional electric rotating machine, and FIG. 12B illustrates the total torque ripple generated in the automotive alternator disclosed in the above patent document. It can be seen from FIGS. 12A and 12B that with the phase difference of π/6 provided between the first and second three-phase stator coils, the total torque ripple generated in the automotive alternator is considerably reduced in comparison with the torque ripple generated in the conventional electric rotating machine.
However, in the automotive alternator disclosed in the above patent document, the first and second three-phase stator coils are wound in a distributed winding manner so that each of the phase windings of the first and second three-phase stator coils is wound across a plurality of stator core teeth. Consequently, it is difficult to suitably arrange the coil ends of the first and second three-phase stator coils (i.e., the axial end portions of the stator coils protruding from the axial end faces of the stator core). As a result, it is difficult to minimize the size of the electric rotating machine.